The present invention generally relates to wafer bonding methods. More particularly, this invention relates to a wafer bonding process that uses a glass frit bonding material to bond a pair of wafers and form a package with an enclosed cavity, wherein the stand-off distance between the wafers is controlled by the bonding material and the width of the bonding surface contacted by the bonding material is minimized to reduce package size.
Within the semiconductor industry, there are numerous applications that require bonding a semiconductor wafer to a second wafer or glass substrate. As an example, a microelectromechanical system (MEMS) device formed in or on a semiconductor wafer (referred to herein as a device wafer) is often capped by a semiconductor or glass wafer (referred to herein as a capping wafer), forming a package that defines a cavity within which the MEMS device is enclosed and protected. Examples of MEMS devices protected in this manner include accelerometers, rate sensors, actuators, pressure sensors, etc. By the very nature of their operation, MEMS devices must be free to move to some degree, necessitating that the seal between the wafers is sufficient to exclude foreign matter from the cavity. Certain MEMS devices, such as absolute pressure sensors, further require that the cavity be evacuated and hermetically sealed. The performance of motion sensors with resonating micromachined components also generally benefit if the cavity is evacuated so that the micromachined components operate in a vacuum. A hermetical seal also ensures that moisture is excluded from the cavity, which might otherwise form ice crystals at low temperatures that could impede motion of the MEMS device.
In view of the above, the integrity of the bond that secures the capping wafer to the device wafer is essential to the performance and life of the enclosed MEMS device. Various bonding techniques have been used for the purpose of maximizing the strength and reliability of the wafer bond, as well as various intermediate bonding materials, including adhesives, solders, and dielectrics such as glass frit. Silicon direct and anodic bonding techniques that do not require an intermediate material have also been used. As would be expected, each of these bonding techniques can be incompatible or less than ideal for certain applications. Silicon direct and anodic bonding methods require very smooth bonding surfaces, and therefore cannot produce a vacuum seal when trench isolation or unplanarized metal crossunders are employed on the device wafer, such as to electrically interconnect a MEMS device to bond pads outside the vacuum-sealed cavity of the package. In contrast, intermediate bonding materials such as glass frit are able to form suitable bonds with deposited layers, runners and other surface discontinuities often found on device wafers.
Glass frit bonding materials used for wafer bonding are often deposited by a screen printing technique, in which case the material is deposited as a paste that contains a glass fit, a thixotropic binder, and a solvent for the binder. The proportions of glass frit, binder and solvent are adjusted to allow screen printing of a controlled volume of the paste on a designated bonding surface of one of the wafers, typically on the capping wafer. After firing to remove the binder and solvent the capping and device wafers are aligned and then mated so that the remaining glass flit particles (bonded together as a result of the firing operation) contact a complementary bonding surface of the second (e.g., device) wafer. The wafers are then heated to melt the glass frit (e.g., about 425xc2x0 C.), so that on cooling the glass frit material resolidifles to form a substantially homogeneous glass bond line between the wafers.
While a certain bond line width is necessary to form a sufficiently strong wafer bond, minimizing the width of the bond line is desirable from the standpoint of reducing the chip size, which in turn enables the maximum number of chips to be fabricated on a wafer slice. The minimum width and volume of a screen printed glass bond line is not typically limited by concerns for bond strength, but by the capability of the screen printing process. Because of an unacceptable variability of screening processes when thin and narrow lines of paste are printed, the volume of glass frit paste printed is typically greater than that required to effect a reliable hermetic wafer bond. To control the xe2x80x9cstand-offxe2x80x9d distance between wafers, the final thickness of the glass bond line may be established by xe2x80x9cstand-offsxe2x80x9d micromachined in one of the wafers. When the capping and device wafers are mated, pressure is applied to bring the stand-offs into contact with the surface of the device wafer, thus physically establishing the wafer spacing. Consequently, both wafers must have surfaces dedicated to accommodating the stand-offs, increasing the chip size. The excess bond material is forced outward relative to the original printed bond line, leading to a relatively wide bond line that must be accommodated by the respective bonding surfaces on the wafers. As a result, relatively wide bond lines and micromachined stand-offs associated with current glass bonding techniques have artificially limited the size to which wafer bonded chip packages can be reduced.
In view of the above, it would be desirable if an improved wafer bonding process were available that could reduce the widths of the wafer bonding surfaces in order to maximize the chip multiple per wafer slice. It would be further desirable if such a process could simplify wafer fabrication while reducing package cost.
The present invention provides a method for glass frit bonding wafers to form a package, in which the width of the glass bond line is minimized to reduce package size. A particular example is the capping of a device wafer to enclose a micromachined sensing structure, such as a MEMS device. The invention entails the use of a glass frit material that establishes the stand-off distance between wafers, instead of relying on discrete structural features on one of the wafers dedicated to this function. In addition, the amount of glass frit material used to form the glass bond line between wafers is reduced to such levels as to reduce the bond line width, allowing the overall size of the package to be minimized. To accommodate the variability associated with screening processes when low volume lines of paste are printed, the invention further entails the use of storage cavities adjacent the bond line to accommodate excess glass frit material without significantly increasing the width of the bond line.
The method of this invention generally entails providing a paste that comprises, in addition to a glass frit material for the glass bonding process, a particulate filler material having a higher melting temperature than the glass frit material and a diametrical dimension corresponding to the stand-off distance desired between the wafers to be bonded. The paste is deposited on a bonding surface of a first wafer so as to define a bond line thereon, and the first wafer is then heated at least sufficiently to remove any volatile constituents of the paste. The first wafer is then mated with a second wafer so that the bond line is between and contacts bonding surfaces of both wafers. As a result of the diametrical dimension of the particulate filler material, the stand-off distance between the bonding surfaces of the wafers is approximately equal to the diametrical dimension of the filler material. The wafers are then sufficiently heated to melt the glass frit material but not the filler material, and then cooled to form a glass bond line between the wafers, at which time the bonding surfaces of the wafers remain approximately spaced apart by the stand-off distance established by the filler material.
According to a preferred aspect of the invention, at least one storage area defined by a cavity, trench, etc., is present in the surface of the first wafer, and the paste is printed on the bonding surface of the first wafer adjacent the storage area so that any excess portion of the paste flows into the storage area during the mating step. The paste is also preferably printed adjacent a peripheral edge of the first wafer, so that any additional excess portion of the paste flows beyond the peripheral edge when the wafers are mated together. The storage area and peripheral edge are preferably defined by respective walls contiguous but not perpendicular to the bonding surface of the first wafer. As a result, the portions of the paste that flow into the storage area and beyond the peripheral edge flow along the sloping walls away from the second die, such that these portions have a relatively large combined volume, preferably greater than the volume of the remainder of the paste remaining between the bonding surfaces, the storage area and the peripheral edge.
Using the method of this invention, the width of a glass bond line can be significantly reduced, thereby reducing the width of the bonding surfaces required to accommodate the bond line. In addition, the fabrication of a device package can be simplified by eliminating the need for micromachined structures to physically establish the stand-off between wafers. As a result, the package size can be reduced and the number of packages that can be fabricated on a given wafer increased. Though minimizing the width of the bond line, the present invention enables a sufficient amount of bonding material to be available to fill any trenches or other surface discontinuities present in the surface of the wafers, while any excess bonding material is accommodated while having minimal impact on the bond line width. Accordingly, the wafer bonding process of the present invention is able to yield a highly reliable package containing a MEMS device that can be hermetically sealed within the package, while also reducing the cost of the package.
Other objects and advantages of this invention will be better appreciated from the following detailed description.